I have hitherto sometimes spoken as if the variations--so common and
multiform in organic beings under domestication, and in a lesser
degree in those in a state of nature--had been due to chance. This, of
course, is a wholly incorrect expression, but it serves to acknowledge
plainly our ignorance of the cause of each particular variation. Some
authors believe it to be as much the function of the reproductive
system to produce individual differences, or very slight deviations of
structure, as to make the child like its parents. But the much greater
variability, as well as the greater frequency of monstrosities, under
domestication or cultivation, than under nature, leads me to believe
that deviations of structure are in some way due to the nature of the
conditions of life, to which the parents and their more remote
ancestors have been exposed during several generations. I have
remarked in the first chapter--but a long catalogue of facts which
cannot be here given would be necessary to show the truth of the
remark--that the reproductive system is eminently susceptible to
changes in the conditions of life; and to this system being
functionally disturbed in the parents, I chiefly attribute the varying
or plastic condition of the offspring. The male and female sexual
elements seem to be affected before that union takes place which is to
form a new being. In the case of "sporting" plants, the bud, which in
its earliest condition does not apparently differ essentially from an
ovule, is alone affected. But why, because the reproductive system is
disturbed, this or that part should vary more or less, we are
profoundly ignorant. Nevertheless, we can here and there dimly catch a
faint ray of light, and we may feel sure that there must be some cause
for each deviation of structure, however slight.

How much direct effect difference of climate, food, etc., produces on
any being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in
that of plants. We may, at least, safely conclude that such influences
cannot have produced the many striking and complex co-adaptations of
structure between one organic being and another, which we see
everywhere throughout nature. Some little influence may be attributed
to climate, food, etc.: thus, E. Forbes speaks confidently that shells
at their southern limit, and when living in shallow water, are more
brightly coloured than those of the same species further north or from
greater depths. Gould believes that birds of the same species are more
brightly coloured under a clear atmosphere, than when living on
islands or near the coast. So with insects, Wollaston is convinced
that residence near the sea affects their colours. Moquin-Tandon gives
a list of plants which when growing near the sea-shore have their
leaves in some degree fleshy, though not elsewhere fleshy. Several
other such cases could be given.

The fact of varieties of one species, when they range into the zone of
habitation of other species, often acquiring in a very slight degree
some of the characters of such species, accords with our view that
species of all kinds are only well-marked and permanent varieties.
Thus the species of shells which are confined to tropical and shallow
seas are generally brighter-coloured than those confined to cold and
deeper seas. The birds which are confined to continents are, according
to Mr. Gould, brighter-coloured than those of islands. The
insect-species confined to sea-coasts, as every collector knows, are
often brassy or lurid. Plants which live exclusively on the sea-side
are very apt to have fleshy leaves. He who believes in the creation of
each species, will have to say that this shell, for instance, was
created with bright colours for a warm sea; but that this other shell
became bright-coloured by variation when it ranged into warmer or
shallower waters.

When a variation is of the slightest use to a being, we cannot tell
how much of it to attribute to the accumulative action of natural
selection, and how much to the conditions of life.